1. Field of the Invention
This invention relates generally to feed forward systems and, in particular, to feed forward systems which control the phase and/or amplitude of the output signal.
2. Description of the Related Art
Feed forward correction, invented by Harold S. Black in 1924, is limited in performance primarily due to variations in system components over power, temperature, and time. Several circuits have been employed in order to limit these variations. These circuits, however, introduce other limitations.
U.S. Pat. No. 5,051,704 issued to Chapman et al., which is incorporated herein by reference, discloses a feed forward amplifier which utilizes a least-means squared circuit in order to detect and cancel system errors. The least-means squared circuit, however, generates internal errors due to leakage signals inherent in the correlators of the least-means squared circuit. These correlators use mixing devices in order to generate an error signal necessary for correction. A typical mixer or Gilbert cell typically passes RF signals from the local oscillator (LO) and RF port to the intermediate frequency (IF) port whose output signal is generally attenuated with respect to the RF port input and varies over time, temperature and power. Variations of the IF output over time, temperature, and power limit the least-means squared circuit's accuracy to detect and cancel system errors. As a result, variations over time, temperature, and power are not entirely eliminated by the above system.
U.S. Pat. No. 5,528,196 issued to Baskin et al., which is incorporated herein by reference, discloses a feed forward amplifier in which a differential phase/amplitude detector is utilized to control the cancellation of the information signals (i.e., the carrier signals) at the output of the summer of the first loop, and an out-of-band pilot (reference) signal is employed in order to control the distortion signals output by the second loop so as to cancel distortion signals at the RF output port. The differential phase/amplitude detector of the first loop requires separate detectors for each of two RF signals that it receives, i.e. the RF signal from the amplifier of the first loop and the RF signal from the RF input port. Any difference in efficiency of these detectors over various conditions results in reduced cancellation of the information signal by the first loop. Furthermore, even when the differential phase/amplitude detector allows cancellation of the information signal at the output of the summer of the first loop, the information signals may leak into and infiltrate the low power stages of the second loop. The pilot signal for the second loop must be located far out of the frequency band of interest, i.e., the frequency band that contains the information signals and their accompanying distortion products, in order to avoid errors caused by interference from the information signals and accompanying distortion products, and to avoid feedthrough of the distortion products to the output. Accuracy of the control system is limited because RF performance of components varies over frequency, thus accuracy out of band does not necessarily imply in-band accuracy of the loop. As the pilot signal is outside the frequency band of interest, it does not necessarily correct for time, temperature, and power variations within the frequency band of interest. Furthermore, as in the first loop, the control system (i.e., the loop 2 amplitude and phase controller) of the second loop utilizes separate detectors for each of its input paths. Any difference in the efficiency of these detectors over various conditions results in reduced cancellation of the distortion signals by the second loop.
As a result, there is a need in the art for an RF amplifier feed forward correction system that provides stable amplification of the input RF signal over variations in power, temperature, and time, while avoiding the aforementioned limitations, as well as others, of prior art systems.